1. Field of the Invention
This invention relates to a non-linear optical device comprising a polymer configured to having long grating holding ratio. More particularly, the polymer comprises at least one repeating unit that comprises a carbazole moiety and at least one repeating unit that comprises a tetraphenyl diaminobiphenyl moiety.
2. Description of the Related Art
Photorefractivity is a phenomenon in which the refractive index of a material can be altered by changing the electric field within the material, such as by laser beam irradiation. The change of the refractive index typically involves: (1) charge generation by laser irradiation, (2) charge transport, resulting in the separation of positive and negative charges, (3) trapping of one type of charge (charge delocalization), (4) formation of a non-uniform internal electric field (space-charge field) as a result of charge delocalization and (5) a refractive index change induced by the non-uniform electric field. Good photorefractive properties are typically observed in materials that combine good charge generation, charge transport or photoconductivity and electro-optical activity. Photorefractive materials have many promising applications, such as high-density optical data storage, dynamic holography, optical image processing, phase conjugated mirrors, optical computing, parallel optical logic, and pattern recognition. Particularly, long grating holding ratio can contribute significantly for high-density optical data storage or holographic display applications.
Originally, the photorefractive effect was found in a variety of inorganic electro-optical (EO) crystals, such as LiNbO3. In these materials, the mechanism of a refractive index modulation by the internal space-charge field is based on a linear electro-optical effect.
In 1990 and 1991, the first organic photorefractive crystal and polymeric photorefractive materials were discovered and reported. Such materials are disclosed, for example, in U.S. Pat. No. 5,064,264. Organic photorefractive materials offer many advantages over the original inorganic photorefractive crystals, such as large optical nonlinearities, low dielectric constants, low cost, lightweight, structural flexibility, and ease of device fabrication. Other important characteristics that may be desirable depending on the application include sufficiently long shelf life, optical quality, and thermal stability. These kinds of active organic polymers are emerging as key materials for advanced information and telecommunication technology.
In recent years, efforts have been made to enhance the properties of organic, and particularly polymeric, photorefractive materials. Various studies have been done to examine the selection and combination of the components that give rise to each of these features. The photoconductive capability is frequently provided by incorporating materials containing carbazole groups. Phenyl amine groups can also be used for the charge transport part of the material.
Non-linear optical ability is generally provided by including chromophore compounds, such as an azo-type dye that can absorb photon radiation. The chromophore may also provide adequate charge generation. Alternatively, a material known as a sensitizer may be added to provide or boost the mobile charge for photorefractivity to occur.
The photorefractive composition may be made by mixing molecular components that provide desirable individual properties into a host polymer matrix. However, most of previously prepared compositions failed to show good photorefractivity performances, (e.g., high diffraction efficiency, fast response time and long-term stability). Efforts have been made, therefore, to provide compositions which show high diffraction efficiency, fast response time and long stability.
U.S. Pat. Nos. 6,653,421 B1 and 6,610,809 B1 disclose (meth)acrylate-based polymers and copolymer based materials which showed high diffraction efficiency, fast response time, and long-term phase stability. The materials show fast response times of less than 30 msec and diffraction efficiency of higher than 50%, along with no phase separation for at least two or three months.
None of the materials described above achieves the desired combination of high diffraction efficiency with a fast response time and long-term stability, along with long grating holding ratio. A material with long grating holding ratio possesses the ability to exhibit grating signal behavior after several minutes of irradiation. Optical devices with these properties are useful for various applications, such as data or image storage. Thus, there remains a need for optical devices comprising materials that combine good photorefractivity performances with long grating holding ratio.